This invention relates to semiconductor materials and, more particularly, to image intensifier devices and apparatus for heat cleaning photoemissive cathodes for use in such devices.
Image intensifier devices multiply the amount of incident light they receive and thus provide an increase in light output which can be supplied to a camera or directly to the eyes of a viewer. These devices are particularly useful for providing images from dark regions and have both industrial and military application. For example, these devices are used for enhancing the night vision of aviators, for photographing extraterrestrial bodies and for providing night vision to sufferers of retinitis pigmentosa (night blindness). These devices enable night time vision by responding to low level radiation which is present at night to enable a user to visually perceive a scene or object.
Image intensifier devices utilize a photoemissive wafer which is bonded to a glass faceplate to form a cathode. Light enters the faceplate and strikes the wafer, thereby causing a primary emission of electrons.
After the formation of the cathode, a heat cleaning step is performed to remove contaminants, such as oxygen and carbon from the surface of the photoemissive wafer. Bringing the cathode to a specific temperature and maintaining the cathode at that temperature are necessary in effecting proper heat cleaning. This must be done, however, without adversely affecting the structure and properties of the photoemissive wafer.
Prior methods of heat cleaning employed tungsten halogen lamps. The principle problem associated with heat cleaning by radiation from a lamp is the difficulty in accurately measuring the temperature of the wafer surface, particularly where the surface is composed of gallium arsenide (GaAs). The measurement of temperature is accomplished by means of a optical pyrometer which measures black body radiation by measuring the peak wavelength being emitted by a body and translating that wavelength into temperature. The pyrometer measures infrared radiation in the 4.8-5.2 .mu.m range. However, radiative type measurements are influenced by factors leading to redistribution of energy and inaccurate temperature measurement such as emissivity or deviation from perfect black body radiation, interference from window layers, window layer materials, stray radiation, etc.
In addition the spectrum of the lamp has a considerable portion of its energy at wavelengths which are transmitted through the faceplate and absorbed in the wafer layers. This causes intense heating of those layers with large thermal gradients due to the fact that the wafer layers absorb heat, whereas the optical material of the cathode faceplate allows most of the heat to pass through it. Hence the temperatures of the glass and the wafer layers may vary by hundreds of degrees centigrade and even the temperatures of the different areas of the wafer itself may vary by tens of degrees. The large difference in temperatures causes stress between the faceplate and the wafer which leads to the formation of brush lines during cooling of the cathode from the heat cleaning temperature.
It is an object of the present invention to provide heat cleaning apparatus which overcomes the disadvantages of the prior art.
It is an additional object of the present invention to provide heat cleaning apparatus which removes contaminants from semiconductor material without adversely affecting the material itself.
It is a further object of the invention to provide heat cleaning apparatus for a cathode which prevents large thermal gradients in the photoemissive wafer.
It is yet another object of the invention to provide a mehtod for heat cleaning a semiconductor material which provides uniform heating of the structure to be cleaned.
It is another object of the invention to provide an apparatus whereby accurate temperature measurement of the surface to be cleaned can be obtained.
These objects and others which will become apparent hereinafter are accomplished by the present invention which provides apparatus for heat cleaning a semiconductor material including means for producing a uniform thermal distribution in the area of the semiconductor material being cleaned; and means for positioning the semicoductor material adjacent said producing means. One feature of the invention includes as the producing means a source of radiation and means for absorbing the radiation emitted from the source and transmitting that portion of the radiation which has a wavelength greater than approximately 1 .mu.m to the semiconductor material for a length of time sufficient to effect the removal of contaminants from the area being cleaned. Another feature of the invention includes a laser and an optics system as the producing means.
The present invention also provides a method of heat cleaning a semiconductor material which includes positioning the semiconductor material in a vacuum chamber; effecting a uniform distribution of heat in a predetermined area of the semiconductor material; maintaining the heat in the predetermined area at a level and for a length of time sufficient to free contaminants from the predetermined area.